The present invention relates to an electromagnetic flow meter having a coil excited by a square wave exciting current or a trapezoidal wave exciting current, and more particularly to an electromagnetic flow meter wherein the excitation frequency and sampling time are improved.
FIG. 1 shows an electromagnetic flow meter similar to one disclosed in U.S. Pat. No. 4,206,641. This flow meter is excited by a square wave current. The flow meter comprises a flow rate detector 1, a pulse width modulation circuit 10 and a pulse-width to voltage converter circuit 30. The pulse width modulation circuit 10 produces a pulse having a width proportional to the voltage of the output signal from the flow rate detector 1. The converter circuit 30 smoothes the pulse from the circuit 10 and convert the pulse into a DC analog signal. The flow rate detector 1 is comprised of a conduit 2, an exciting coil 3 and a pair of electrodes 4. These electrodes 4 mutually face and are arranged on the inner periphery of the conduit 2, through which a fluid flows and which magnetic fluxes generated by the coil 3 penetrate at right angles to the flow of the fluid. Two constant current sources 13, 14 are alternately connected to the exciting coil 3 by a switch 12 which is changed over by a switch control signal Ea shown in FIG. 2(a). Hence, the coil 3 is excited by a current Eb having the waveform as shown in FIG. 2(b). When an electrically conductive fluid flows through the conduit 2 under this condition, an electromotive force, which is proportional to the intensity of the magnetic field and the speed of the fluid, will be generated between the electrodes 4 in accordance with Fleming's right-hand rule. This electromotive force is picked up by the electrodes 4 and then amplified by an AC amplifier 15. The output signal Ec of the AC amplifier 15, which is shown in FIG. 2(c), is applied to an inverting amplifier 26 the amplification factor of which is 1. The inverting amplifier 26 inverts the signal Ec. The output signal Ec of the AC amplifier 15 and the output signal of the inverting amplifier 26 are so selected by a switch 16 that a negative voltage is applied to a switch 17 at all times. The switch 16 is changed over by the switch control signal Ea from a timing control circuit 11.
The output signal Ed of the switch 16, which is shown in FIG. 2(d), is supplied to a double integration circuit, which will be described later.
The switch 17 is turned on by the signal Ee from the timing control circuit 11 during the one-cycle period of the commercially available AC current, when the magnetic fluxes become stable, that is, immediately before the switch 12 is changed over. Hence, the switch 17 samples the signal Ed, i.e., the electromotive force, to thereby produce a flow rate signal not containing so-called 90.degree.-shifted noise. A double integration circuit, which comprises resistors 18, 19, a capacitor 20 and an operational amplifier 21, integrates the signal Ed at regular intervals, each time for one-cycle period of the commercially available AC current, thus producing a signal Ef which contains no noise as shown in FIG. 2(f). Then the output signal Ef from the double integration circuit is supplied to a comparator 22. The output signal from the comparator 22 falls from the logical "1" level to the logical "0" level when the signal Ef falls to the zero level. The output signal of the comparator 22 resets a flip-flop 23, and the signal Ea from the timing control circuit 11 sets the flip-flop 23. As a result, the flip-flop 23 generates a pulse signal Eg whose width is proportional to the level of the flow rate signal, i.e., the output signal of the comparator 22.
As shown in FIG. 1, the pulse width modulation circuit 10 further comprises a reference power source 24, a switch 25 which is turned on and off by the output Q of the flip-flop 23, and a light emitting element 31.
The pulse signal produced by the flip-flop 23 is supplied to the pulse-width to voltage converter circuit 30. More specifically, upon receipt of the pulse signal from the flip-flop 23, the element 31 emits light, which reaches a light-receiving element 32 electrically insulated from the element 31. The element 32 generates an output signal, which turns a switch 33 on and off. The current supplied from a power source 34 to the switch 33 is therefore made into a pulse signal. This pulse signal is smoothed by a smoothing circuit comprised of a resistor 35 and an operational amplifier 37. The output signal from the operational amplifier 37 is supplied to an output circuit 38. The output circuit 38 converts the input signal to a DC analog signal of, e.g., 4 to 20 mA. The power source 34 and amplifier 37 are grounded at a point other than the point at which the amplifier 21, comparator 22, power source 24 and element 31 are grounded.
FIG. 3 is a circuit diagram of the timing control circuit 11 for controlling the switches 12, 16 and 17. In this circuit 11, an output signal Eh of a commercially available power source 111, whose waveform is shown in FIG. 4, is supplied to a buffer circuit 112, which generates a pulse signal Ei shown in FIG. 4. The frequency of the pulse signal Ei is divided by D-type flip-flops 113 to 115, whereby a signal El shown in FIG. 4 is obtained. This signal El is used to control the switches 12 and 16. The output signals from the terminals Q of the D-type flip-flops 113, 114, i.e., the signals Ej, Ek shown in FIGS. 4, are supplied to a two-input AND gate 116, which produces an output signal Em shown in FIG. 4. This signal Em is used to control the switch 17. The circuit 11 shown in FIG. 3 can therefore easily generate a pulse signal which is at the high level only for the one-cycle period of the commercially available AC current immediately before the switch 12 is changed over.
The pulse widths of the signals El and Em from the timing control circuit 11 change in accordance with the frequency of the commercially available AC current, 50 Hz or 60 Hz. The timing for sampling the flow rate signal will inevitably change also in accordance with the frequency of the AC current. Although the exciting current is constant, the output signal of the pulse-width to voltage converter circuit 30 will change, though very little, due to the transient phenomena occuring when the switch 12 is changed over, e.g., the deterioration of the responses of the detector 1 and circuits 10, 30, even if the flow rate of the fluid remains unchanged. Why this takes place will be explained with reference to FIG. 5.
In FIG. 5, curve a indicates the waveform of an exciting current of 60 Hz and curve b represents an exciting current of 50 Hz. The horizontally hatched region c represents the sampling period when the excitation current of 60 Hz is supplied to the coil 3. The diagonally hatched region d indicates the sampling period when the exciting current of 50 Hz is supplied to the coil 3. It is clear from FIG. 5 that the current of 60 Hz and the current of 50 Hz are different in magnitude. In other words, the flow rate signal resulting from the current of 60 Hz supplied to the coil 3 is different in magnitude from the flow rate signal resulting from the current of 50 Hz supplied to the coil 3 even if the flow rate of the fluid remains unchanged. When the electromagnetic flow meter is used to detect the flow rate with a high precision, the amplification factor of the circuit 10 or the magnitude of the exciting current must be adjusted in accordance with the frequency of the exciting current. In practice, the amplification factor of the circuit 10 is fixed since it is not economical to use two types of circuits 10, one for 50 Hz AC current and the other for 60 Hz AC current. Hence, it is the magnitude of the exciting current that is changed in accordance with the frequency of the exciting current, 60 Hz or 50 Hz.
The electromagnetic flow meter must therefore be adjusted so that it may be used in a specific region or country, in accordance with the frequency of the AC current which is commercially available in the region or country. This adjustment is troublesome and time-consuming and undesirable from an economical point of view. To put the flow meter to practical use without adjusting it for this purpose, the responses of the flow rate detector 1 and pulse width modulation circuit 10 may be improved. To improve the response of the detector 1, that is, to stabilize the magnetic fluxes quickly, the detector 1 may be so designed that no eddy current flows through it. Alternatively, the voltages applied to the exciting circuit, i.e., the combination of the coil 3 and electrodes 4, and constant current sources 13, 14 are elevated for the same purpose. The first and second methods, however, would require a more complicated and expensive flow rate detector and a larger exciting circuit.
To improve the responses of the detector 1 and circuit 10 is to broaden the frequency band of the circuit 10. In view of the fact that the electromotive generated between the electrodes 4 is small, it is not advisable to broaden the frequency band of the circuit 10 since this method will reduce the S/N ratio of the output signal from the circuit 10. Indeed the noise may be removed from the output signal from the circuit 10 if the switch 17 is on for 100 msec each time, whether the frequency of the commercially available AC current is 50 Hz or 60 Hz. Further, the frequency of excitation may be fixed to raise the S/N ratio of the output signal. Either method, however, will deteriorate the response of the electromagnetic flow meter since the frequency of excitation must be considerably low.